We stated earlier that biology is the study of life. But what exactly is life? As simple as this question seems, it is frustratingly difficult to answer. We all recognize life when we see it, but coming up with a definition is harder than it first appears.

Living organisms are clearly different from nonliving things. But just how different is an organism from the rock shown in Fig. 1.4? On one level, the comparison is easy: The rock is much simpler than any living organism we can think of. It has far fewer components, and it is largely static, with no apparent response to environmental change on timescales that are readily tracked.

In contrast, even an organism as relatively simple as a bacterium contains many hundreds of different chemical compounds organized in a complex manner. The bacterium is also dynamic in that it changes continuously, especially in response to the environment. Organisms reproduce, which minerals do not. And organisms do something else that rocks and minerals don’t: They evolve. Indeed, the molecular biologist Gerald Joyce has defined life as a chemical system capable of undergoing Darwinian evolution.

From these simple comparisons, we can highlight four key characteristics of living organisms: (1) complexity, with precise spatial organization on several scales; (2) the ability to change in response to the environment; (3) the ability to reproduce; and (4) the capacity to evolve. Nevertheless, the living and nonliving worlds share an important attribute: Both are subject to the basic laws of chemistry and physics.